The present invention relates to optical scanners, and more particularly, to optical scanners having a plurality of photosensors mounted in a linear array past which a document is fed.
Optical scanners for reading text and graphics off of documents are in widespread use in facsimile machines, digital copiers, and in portable and flatbed scanners used with personal computers. The image data produced by these devices is used to transmit information in the case of a facsimile machine, make additional copies in the case of a digital copier, and to store, display and manipulate the same in the case of a personal computer.
A typical optical scanner includes an optical imaging assembly comprising illumination, optical and detection systems. The illumination system includes a light source that illuminates a portion of the object which is commonly referred to as a scan region. The optical system collects the light reflected by the illuminated scan region and focuses a small area of the illuminated scan region, commonly referred to as the scan line, onto the surface of the detection system that typically comprises, for example, a photosensor module positioned within the scanner. The photosensor module converts the image light incident thereon into electrical signals representative of the scan line. Image data representative of the entire document may be obtained by sweeping the scan line across the entire document.
Facsimile machines and other low cost xe2x80x9cscroll fedxe2x80x9d optical scanners typically use a contact image sensor (CIS) which is a type of photosensor module that is smaller than optical reduction systems. The photosensors in a conventional CIS are either charge coupled devices (CCDs) or CMOS devices. They are arranged in a linear array and are spaced at the pitch of the scanner""s resolution, e.g. three hundred photosensors per inch for a three hundred dots per inch (DPI) scanner. Each of these photosensors must be calibrated before a high-quality scan operation can be performed. Photosensors are calibrated by imaging a target with known color properties, which is typically a white reference surface, and applying a gain to the output of each photosensor such that the signal returned matches the expected signal for the white reference surface.
Scroll fed scanners that employ a CIS module inherently build up debris on the white reference surface as the module contacts the paper fibers, roller debris, and other artifacts that may be fed into the scanner along with the original documents. This debris often creates annoying vertical streaks along the entire vertical dimension of the copy or scan output. It is possible to detect low photosensor values in the white reference scan data. However, these low values may be due to debris on the white reference surface, weak photosensors, debris within the CIS module, or combinations of the same.
In prior art scroll fed scanners, when the white reference surface contains debris and a calibration is performed, gains will be set erroneously. For example, assume that the average white reference photosensor value returned before gains are applied to each photosensor is one hundred. During calibration, a photosensor imaging debris on the white reference surface may have a value of fifty. The gain for this photosensor will then be set twice as high as that of the other photosensors. When a document scan is subsequently performed, the higher signal from the photosensor that was calibrated while imaging debris will result in a bright vertical streak in the copy or scan output. Clearly, in this situation an additional gain should not have been applied to the photosensor which returned a low white reference scan value due to the existence of debris on the white reference surface.
Alternatively, the CIS module may contain a weak photosensor or may have debris internal thereto. Assume once again that the average white reference photosensor value returned before gains are applied to each photosensor is one hundred. For a weak photosensor, or a photosensor being blocked by debris within the CIS module, the value may be fifty, for example. In either of these cases, the gain for this photosensor will be set twice as high as that for the other photosensors. When a document scan is subsequently performed, the gain for the weak or obstructed photosensor will be corrected so that the quality of the copy or scan output will be acceptable, i.e. there will be no streaks.
Flatbed scanners can average out a weak response from a photosensor due to debris by moving the scanner head or bar during the white reference calibration scan. However, heretofore prior art scroll fed scanners have not adequately taken into account all of the variables when calibrating the photosensors against the white reference surface, resulting in undesirable streaking in the copy or scan output. Accordingly it would be desirable to provide an improved scroll fed scanner that would taken into account debris and other artifacts, as well as low output photosensors, to ensure optimum copy or scan output.
Therefore, it is the primary object of the present invention to provide an improved scroll fed scanner that takes into account the existence of debris and other artifacts, as well as low output photosensors, in order to ensure optimum copy or scan output.
It is another object of the present invention to provide an improved method of adjusting photosensor output in a scroll fed optical scanner to eliminate streaks in the scan output.
In accordance with the present invention an optical scanner includes a scroll fed transport for propelling a document to be scanned along a paper path including an optical reference surface. A light source illuminates the optical reference surface, or if a document is being propelled along the paper path over the optical reference surface, a scan region on the document. A plurality of photosensors receive light reflected from the optical reference surface or the scan region on the document. A circuit connected to the photosensors generates image data representative of information printed or otherwise formed on the document and adjusts the gains applied to the outputs of selected ones of the photosensors to eliminate streaks in the image data otherwise due to the selected photosensors imaging debris on the optical reference surface.
The present invention also provides a method of ensuring optimum scan output quality of a scanner. An initial factory calibration scan of a reference surface is performed with a plurality of photosensors when the white reference surface is known to be clean The locations of all photosensors having output values below a first predetermined threshold value are stored in a memory. Thereafter, a subsequent user environment calibration scan of the reference surface is performed. The gain applied to each photosensor output is adjusted if its output value falls below a second predetermined threshold value, but only if its location was not one of the locations stored during the initial calibration scan. The amount of gain adjustment is sufficient such that the scan output is devoid of visible streaks otherwise due to the photosensors imaging debris or other artifacts present on the reference surface during the subsequent user environment calibration scan.